Thus, topsoil does not accumulate like most sediment, by simply piling up.
I suspect that most of them belong to plants which were chopped down years ago.
There's not much down there in that clay to completely rot them away.
The sediment added to our patch of land may be great for building new soil, but if it accumulates too quickly it will merely bury the existing soil. In any case, the old topsoil, now compressed and deeply buried by sediment and soil, is no longer turned over by earthworms or small animals.
It is deprived of oxygen and fresh organic material, such as rotting leaves.
(Topsoil is full of microbes that love to munch away on organic material, and don't forget the earthworms.
Those earthworms don't get their calories from rock and clay!
As more and more of that rock is weathered by the mechanical effects of freezing and thawing, the chemical and mechanical action of roots, or by other means, the soil is deepened.
However, the deeper that soil gets, the more insulated the parent rock becomes to weathering.
Whatever damage is done to the clay by the few penetrating roots may, for all I know, be patched up by clay particles sifting down through the soil.